This invention relates to burners. In one aspect this invention relates to oil burners while in another aspect, this invention relates to oil burners equipped with an air atomization system. In yet another aspect, this invention relates to oil burners in which the air atomization system includes a rotary compressor.
Many burners, i.e. devices designed to produce heat from the combustion of hydrocarbon products or derivatives of hydrocarbon products, are designed to burn oil and of these burners, many are designed to burn more than one grade of oil. Typical of these burners are the scotch or fire box fire tube boilers, cast iron boilers, water tube boilers, air heaters, and dryers. These burners are equipped with various means for delivering the oil from a holding or storage tank to the burner, and for delivering air to the burner for combustion with the oil. The means for delivering the oil from the storage tank (often located a significant distance from the burner) to the burner typically includes a pump and a series of transfer lines, check valves and automatic and manual valves. The means for delivering the combustion air to the burner typically includes an electric motor driven impeller.
Fuel oil does not burn in the liquid state. To be combustible, it must be atomized and intimately mixed with air. Atomization is the process in which a liquid is converted to a spray or to an aerosol with mechanical energy rather than heat. The latter vaporizes the fuel and as such, it is usually used only with low boiling fuels, e.g. gasoline, kerosene, alcohol, fuel oil #1, etc., in gasoline engines and relatively small burners.
Fuel oil of No. 2 grade or heavier can be mechanically atomized by spinning it from the edge of a rapidly rotating cup or disc or by discharging it at high velocity through a nozzle. Pressure atomizing through a nozzle produces a conical spray of fine droplets. These droplets disperse and support themselves solely by kinetic energy (velocity). To burn, these droplets must be mixed with air and heated to their ignition temperature.
Pressure atomized spray droplets rapidly lose velocity after leaving the nozzle due to air friction. Air resistance limits the ability of the droplets to remain in suspension. High atomizing pressure, e.g. pressure in excess of 10 psi, is required to produce fine droplets and aid mixing with secondary air.
Air atomization produces an aerosol in which the fine droplets are supported by an expanding cone of air. An aerosol is a gaseous suspension of fine solid or liquid particles, as opposed to a spray which is a liquid moving in a mass of dispersed droplets. Since the volume of atomizing air is constant regardless of fuel viscosity or oil flow rate, the aerosol cone maintains essentially the same size and shape, regardless of the amount of entrained oil. This permits sizable turn down ratios without change in size or shape of the aerosol cone.
Aerosol droplets moving at the same velocity as the propelling air are not affected by friction. The flame cone retains the same shape at all firing rates. Since the low fire flame is simply shorter, but essentially the same diameter, aerosol mixing with the secondary air is as effective at low fire as at high fire. Conversely, pressure atomized spray, being smaller in diameter at low fire does not mix with the secondary air as effectively and the outer envelope of the secondary air may even completely escape the fuel cone. As such, the turn down ratios with mechanical atomization are much more limited than with air atomization.
Since air supply is an important component of efficient oil burner operation, an integral air compressor is an important component of the oil burner. When properly matched to the characteristics of the burner nozzle, the compressor delivers atomizing air to the burner in an amount sized to the amount and quality of oil which in turn allows the burner to operate at high efficiency.
The compressor is usually one of three types, reciprocating (either single or multiple cylinder), screw or rotary. While reciprocating and screw compressors work well, their cost is often two or three times that of a similar size rotary compressor and as such, rotary compressors are generally favored over reciprocating and screw compressors. However, rotary compressors must be well lubricated to avoid excessive wear and traditionally, these compressors have been equipped with an independent lubricating system. These systems typically comprise a holding tank for the lubricating oil, transfer lines from the tank to the compressor, an oil filter, a separator to remove air from the oil, and a heat exchanger to remove the heat that the oil acquired from the compressor.
Because the lubricating oils enter the compressor by way of a relatively small aperture, e.g. 1/32nd or 1/64th inch in diameter, these oils must be of low viscosity (but liquid) and free of particulate matter. Accordingly, these oils usually pass through a filter and are cooled (by way of the heat exchanger referred to above) before entering the compressor. This system adds to the capital and operational costs of the burner.